1. Technical Field
The technical field is ceramic capacitors. More particularly, the technical field is ceramic capacitors that may be embedded in printed wiring boards (PWBs).
2. Relevant Art
Passive circuit components embedded in printed wiring boards formed by “fired-on-foil” technology are known. Fired-on-foil capacitors are formed in the following way: a thick-film dielectric material layer is deposited onto a metallic foil substrate; a thick-film top electrode material is deposited over the thick-film dielectric material layer; and the thick-film dielectric and electrode are fired under copper thick-film firing conditions, such as 900° C. for 10 minutes under a protective atmosphere. U.S. Pat. No. 7,072,167 to Borland discloses such a process.
Copper foil and copper thick-film conductive compositions are known materials in making fired-on-foil capacitors because copper is compatible with printed wiring board processes. The thick-film dielectric material should have a high dielectric constant (K) after firing. A high dielectric constant thick-film dielectric is formed by mixing a high dielectric constant powder (high K functional phase), such as barium titanate, with a glass powder and suitable dopants and dispersing the mixture into a thick-film screen-printing vehicle.
During firing of the thick-film dielectric material, the glass component of the dielectric material softens and flows before the peak firing temperature is reached. The glass encapsulates the high K functional phase, facilitates the incorporation of the dopants into the crystal structure and bonds the dielectric to the copper foil and to the copper top electrode.
Being a rigid substrate, the copper foil essentially eliminates the firing shrinkage of the dielectric in the x-y direction. This means that during firing on copper foil of the dielectric and the top electrode, shrinkage occurs only in the z direction. And, precisely in order to achieve high density of the dielectric by shrinkage in the z direction alone, typical thick-film dielectric compositions are designed to contain substantial amounts of glass, such as greater than 40% by weight of the inorganic composition.
Such substantial amount of glass facilitates densification by forming a liquid phase. At concentrations such as 40% by weight of the total inorganic content, the functional phase, dopants and glass mixture are together sufficiently fluid to easily flow and contract in the z direction to full density. However, this amount of glass severely dilutes the high K phase, resulting in a significantly reduced dielectric constant. This dilutive effect introduces a balancing tension between that amount of glass added to the dielectric for achieving high densification versus that amount added to the dielectric for minimizing reduction of the dielectric constant.
Thus, when the glass content is minimized, typically to less than 20% by weight of the total inorganic composition and preferably much less than that, to achieve satisfactory composite dielectric constants, the functional phase, dopants and glass mixture has a very high viscosity and does not easily flow. Therefore little contraction in the z direction occurs, resulting in incomplete densification of the dielectric. This compromises the long-term reliability of the capacitor.
Consequently, a problem that remains to be solved in fired on foil capacitors is the use of a minimal amount of glass to maintain a high dielectric constant and the creation of a method to minimize the amount of added glass while achieving substantially complete densification of the dielectric.